The present invention is directed to a positive displacement flow through fluid pump and more specifically to a pump especially adapted for photoresist where exact volumes must be dispensed.
Reciprocating bellow type pumps of the nonflow through type have been used for several years. These pumps have a single input/output port on the top of the bellows which is used for dispensing accurate volumes of, for example, photoresist liquid onto silicon wafers. Their disadvantages are a large stagnant volume which must periodically be flushed, the necessity for priming before initial operation and the difficulty of switching from one type of liquid to another without extensive cleaning, etc.
A type of flow through pump has been in use by at least two manufacturers which while eliminating a stagnant volume still has a large static volume of liquid in the pump. And since photoresist fluid is relatively expensive this large amount of static volume is expensive when changing fluids. In addition, these pumps still have the same difficulty as a nonflow through bellows type pump where the convolutions of the bellows have deposited on them the liquid which is being pumped which can only be removed by time consuming cleaning.
Yet another difficulty in both the foregoing types of pumps is that the bellows type structure on the output displacement stroke could be overstressed by too great an upstroke.